Michael Martin, the Speaker of the House of Commons, will face an inquiry into claims that his wife spent more than £4,000 of public money on taxis for shopping trips.

It emerged this evening that John Lyon, Westminster’s Commissioner for Standards, is to investigate Mr Martin’s expenses after a complaint was made by the Tax Payers’ Alliance.

Mr Martin, a former Labour backbencher, has led a campaign to keep MPs expenses secret despite recent pressure for politicians to make their financial dealings more transparent. Mr Lyon has now decided that the Speaker’s own expenses claims deserve closer scrutiny.

In a letter to the Tax Payers’ Alliance, the Commissioner wrote: “In essence, the part of your complaint I have accepted is that Mrs Martin’s expenditure on taxis may not have been in accordance with the Code of Conduct for Members of Parliament and its associated rules.

“I am inviting Mr Speaker to let me have his comments on your complaint. Once I receive his response, I shall consider how to proceed.”

A spokeswoman for the Commissioner’s office confirmed that a preliminary investigation had begun.

The Speaker’s spokesman, Mike Granatt, resigned for ethical reasons in February after admitting that he inadvertently misled a journalist over Mary Martin’s taxi trips to buy food for “entertaining official visitors”.

Mr Granatt had said Mrs Martin was accompanied by a Commons official, but in fact she was with her housekeeper. The excursions landed the public purse with a bill for more than £4,000.

News of the probe will increase the pressure on Mr Martin, who has been repeatedly criticised over his own use of public funds.

This weekend it emerged that more than £700,000 has been spent improving his Westminster residence since 2001.

Questions have also been raised over why he has claimed more than £75,000 in MPs’ allowance for a second home, despite having no mortgage on the Glasgow property he was claiming for.

While he has been under personal attack, Mr Martin has been determination to block the publication of details of allowances granted to all MPs.

Last week, the Commons authorities made the surprise decision to appeal against a ruling by the Information Tribunal that a breakdown of MPs’ spending on second homes should be released.

Members can claim up to £23,000 a year for mortgage interest payments, new bathrooms and kitchens, and even stereos under the so-called “John Lewis list”.

Mark Wallace, campaign director of the Tax Payers’ Alliance, said it was now “completely inappropriate” for the Speaker to remain in charge of the Commons inquiry into MPs’ expense arrangements, which is due to report in the coming months.

“Now the Speaker is being investigated over his household expenses it would be completely inappropriate for him to remain in charge of the Parliamentary expenses system.

“He should hand over control of the review into MPs’ expenses to someone independent and untainted by scandal.”

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